1. Field of the Invention
The present invention relates to a cathode active material for a lithium secondary battery, a method of fabricating the same, and a lithium secondary battery including the same. More specifically, the present invention relates to a cathode active material for a lithium secondary battery, in which a voltage drop of the lithium secondary battery may be reduced by optimizing a composition ratio of lithium to transition metals in a layer-structured fluorine-doped excess-lithium rich-manganese lithium composite transition metal oxide, a method of fabricating the same, and a lithium secondary battery including the same.
2. Discussion of Related Art
As information technology (IT) is gradually developed, capacity and lifespan of a lithium ion secondary battery are also being improved, which is a development of a cell design based on an existing material, LiCoO2. However, a high capacity battery, which has been developed based on the cell design, has reached a capacity limit for use in recent smart apparatuses, electric vehicles, and the like. Accordingly, a need for a new material to be used in the lithium secondary battery is being raised. Recently, studies on layer-structured lithium composite transition metal oxides including excess lithium are being actively conducted since the capacity of a lithium secondary battery largely depends on a cathode active material.
As the cathode active material, lithium-containing cobalt oxide (LiCoO2) is mainly being used, and other lithium-containing manganese oxide, such as layered crystal structured LiMnO2 and spinel crystal structured LiMn2O4, and lithium-containing nickel oxide (LiNiO2) are being considered to be used.
Among the above-described cathode active materials, LiCoO2 is most commonly used as the cathode active material due to excellent lifespan characteristics and excellent charge and discharge efficiency. However, since LiCoO2 has poor structural stability and poor price competitiveness due to limits in resource of cobalt used as a raw material, there is a limit to use LiCoO2 as a power source in a field such as electric vehicles in large quantities.
LiNiO2-based cathode active materials have relatively low prices and high discharge capacities, but have problems in that a rapid phase transition in a crystal structure may occur according to a volume change accompanying charging and discharging cycles, and stability may be sharply lowered when the LiNiO2-based cathode active materials are exposed to air and moisture.
In addition, a lithium-containing manganese oxide such as LiMnO2 has excellent thermal stability and a relatively low price, but has problems, such as small capacity, poor cycle characteristics, and poor high temperature characteristics.
In the case of a spinel-based lithium manganese oxide among the lithium manganese oxides, a relatively flat electrical potential may show in a 4 V zone (3.7 V to 4.3 V) and a 3 V zone (2.7 V to 3.1V) and a large amount of theoretical capacity of about 260 mAh/g or more may be obtained when both two zones are used (The theoretical capacity is about 130 mAh/g in both 3V and 4V zones). However, since cycle and storage characteristics are significantly dropped in the 3V zone, utilization of the spinel-based lithium manganese oxide may be difficult. In addition, when the spinel-based lithium manganese oxide is used alone as the cathode active material, only a half of available capacity may be used since there is no lithium source available for charging and discharging operations in the 3V zone under the present lithium secondary battery system in which the lithium source depends on the cathode active material. Further, since the spinel-based lithium manganese oxide undergoes a rapid voltage drop between the 4V and 3V zones and thereby shows a discrete voltage profile, a problem such as an insufficient output may occur in the 4V and 3V zone. Accordingly, it is practically difficult to use the spinel-based lithium manganese oxide as a power source of a middle- or large-sized device in the field of electric vehicles or the like.
A layer-structured lithium manganese oxide has been proposed in order to overcome the above-described shortcomings of the spinel-based lithium manganese oxide and ensure excellent thermal stability of manganese-based active materials.
In particular, a layer-structured xLi2MnO3.(1-x)LiMO2 (0<x<1 and M=Co, Ni, Mn, etc.) in which a content of manganese (Mn) is greater than other transition metal(s), has a very large capacity when it is overcharged at a high voltage. However, there is a problem in that initial irreversible capacity is large.
In the layer-structured lithium composite transition metal oxide in which an equivalence ratio of lithium to composite transition metals (at least two selected from the group consisting of Ni, Mn, and Co) is one, each element may form LiMO2 (M: at least two transition metals having oxidation numbers of +3 and +4) in a regular structure. However, a lithium composite transition metal oxide having an equivalence ratio of lithium to composite transition metals greater than one may form a repetitive crystal structure of a lithium layer, an oxygen layer, a transition metal layer, an oxygen layer, and a lithium layer. Li2M′O3 (M′: a transition metal having oxidation numbers of +4 such as Mn and Ti) may be formed in such a manner that lithium occupies some sites of the transition metal layer. Since Li2M′O3 has a higher Li content than LiMO2, high capacity may be implemented. However, Li2M′O3 may need to be charged and discharged at a voltage of 4.4 V or more since it is not activated at a voltage of less than 4.4 V. When Li2M′O3 is charged and discharged at a voltage of 4.4 V or more, voltage drop problems may continue to occur in the lithium secondary battery since at least 50% Li is desorbed from LiMO2 and, at the same time, the transition metal is eluted.
Accordingly, needs for a lithium secondary battery having a high capacity and no rapid voltage drop zone, that is, having improved stability by showing an even profile across an entire state of charge (SOC) zone are increasing for use in the power of the middle- or large-sized devices.